Unlocking stable intermediate states in SrFeO_3-δ through voltage control of oxygen non-stoichiometry

Voltage-controlled ion insertion provides a powerful strategy for the analog tuning of material properties, enabling adaptive devices such as neuromorphic transistors and smart displays. Among tunable materials, mixed ionic-electronic conducting oxides undergoing topotactic phase transitions are particularly compelling due to their dramatic property changes between fully oxidized and fully reduced states. However, intermediate oxidation states remain largely underexplored because of significant control limitations. In this work, we investigate the topotactic phase transition in strontium ferrite (SrFeO_3-δ) thin films by progressively and precisely modulating and quantifying oxygen non-stoichiometry via solid-state electrochemical pumping. This fine-tuning approach unveils the co-existence of multiple stable phases in equilibrium configurations across a broad range of oxidation states. A crystallographic mixing model that captures the structural-electronic coupling underlying this phenomenon is proposed, complemented by a defect chemistry framework that quantitatively describes the oxidation mechanism under applied voltage. These findings highlight the critical role of intermediate states in governing functional properties and open new pathways for designing advanced ionotronic oxygen-responsive devices.